Reciprocating internal combustion engines have been equipped with turbochargers. The turbine of a turbocharger is driven by the energy of exhaust gases discharged from an internal combustion engine so that the rotating force is developed for driving the blower which is coaxial with the turbine. The blower compresses the air and the compressed air is directed to the intake system of the engine so that the output may be increased.
The prior art supercharging systems are shown in FIGS. 1 through 3 in which the same reference letters are used to designate similar parts.
The supercharging system shown in FIG. 1 has been widely used. The exhaust gases from an engine a flows through an exhaust pipe d to a turbine b and expands in the turbine b so that work is done and a blower c spins. The blower c discharges the compressed air which in turn is charged through an intake passage e to the engine a. In the supercharging system of the type described above, the turbine casing is of the fixed nozzle type so that from the standpoint of the strength of the engine a, the supercharging pressure is limited to a certain level. As a result, the turbocharger is so designed that a matching point is reached at high speeds. The use of the turbocharger in other speed regions results in insufficient boost or air volume so that the engine performance drops.
In the prior art supercharging system of the type shown in FIG. 2, in order to overcome the defects of the supercharging system described in conjunction with FIG. 1, the matching point between the engine a and the supercharging system is selected at low and medium speeds. When the flow rate of exhaust gases inreases and subsequently the supercharging pressure rises too high at high speeds, part of the exhaust gases are discharged into the surrounding atmosphere through a bypass passage g which is interconnected between the inlet and outlet of the turbine b so as to bypass it and has a waste gate f, whereby the damages to the engine a due to the excessive boost at high speeds can be avoided. With this system, the exhaust gases are discharged into the surrounding atmosphere by bypassing the turbine b, so that the energy losses are high and the engine performance drops at high speeds.
The prior art supercharging system shown in FIG. 3 is equipped with vane nozzles h adapted to adjust the gas inlet angle so that a constant degree of boost can be maintained without causing the exhaust gases to bypass the turbine b. The system for controlling the angle of the vane nozzles h is very complex in construction so that the supercharging system becomes very expensive. In order to overcome this defect, there has been proposed a system in which scrolls of vaneless nozzles are switched. However, this system generally becomes the static pressure type, so that it becomes impossible to utilize low-velocity pulses. In addition, exhaust interference results so that the engine performance drops.
In general, multi-cylinder reciprocating internal combustion engines equipped with turbochargers are easily susceptible to exhaust interference. So, upon defining an exhaust manifold, care must be taken so as not to drop the engine performance. In order to overcome this problem, which occurs especially when the boost is lower than the exhaust gas pressure or the engine is partially loaded, there has been proposed a supercharging system in which the exhaust manifold to the turbine wheel is divided into two or more branches so that cylinders in adjacent explosion order will not exhaust into the same branch and consequently the exhaust interference can be avoided.
According to the present invention, in order to avoid exhaust interference in a scroll when a vaneless variable nozzle is used, the exhaust gases from the engine are directed to flow toward the turbine wheel through a first or radially inward scroll which is substantially divided into two passages in the axial direction of the turbine wheel, whereby the effective utilization of exhaust pulses can be insured. When the flow rate becomes high at high rotational speeds, the pulsations of exhaust gases are reduced and the flow of the exhaust gases reaches almost the steady state. In addition, the boost pressure becomes higher than the exhaust gas pressure. As a result, the adverse effects of exhaust interference are reduced. Therefore, according to the present invention, the use of a second or radially outward scroll defining a single passage throughout its whole length and therefore exhibiting less resistance to the flow of the exhaust gases is very advantageous from the standpoint of turbine efficiency. Thus, the turbine casing in accordance with the present invention is provided with the second or radially outward scroll in addition to the first or radially inward scroll so that when the flow rate of the exhaust gases from the engine becomes high, the gases are automatically directed to flow into not only the first or radially inward scroll but also into the second or radially outward scroll. In other words, the scroll in the turbine casing is increased in volume. In addition, the first and second scrolls are so combined as to coact in such a way that no drop in turbine efficiency will occur under any operating conditions of the engine.
The primary object of the present invention is therefore to provide a turbine casing for turbochargers which is very simple in construction yet capable of maintaining the supercharging efficiency at a predetermined level and utilizing the energy of the exhaust gases regardless of the variation from a low flow rate to a high flow rate of the exhaust gases as the speed of the engine varies from a low speed to a high speed.
Another object of the present invention is to provide a scroll structure which can provide a scroll adapted for a low flow rate when the flow rate of the exhaust gases is low and the pulsations of the exhaust gases is high, but provides a scroll having a large cross section in addition to the first-mentioned scroll when the flow rate of the exhaust gases is high and consequently the steady flow is reached.
The present invention will become more apparent from the following description of some preferred embodiments thereof taken in conjunction with the accompanying drawings.